Over the last years battery powered applications (like smartphones and tablet PCs) increased their computing power, screen resolution and display frame rate and added connected standby modes. This caused an increased drain of the battery, so that many modern smartphones need to be re-charged on a daily basis. Limited mobility time may be addressed by using battery packs with an increased capacity, but re-charging such battery packs typically requires an increased amount of time. This is due to the fact that smartphones are typically charged through a standard (Micro) USB port, which provides limited current handling capability (˜1.5 A). Therefore a 5 Ah battery pack requires several hours for re-charging, even through the battery technology (typically LiIon/LiPolymer) would allow for increased charging power levels (e.g. 1-2 C charging).
Recent changes in the USB charging specification allow for higher voltages than the standard 5V, enabling up to 4× the power from the USB supply (9V, 12V and 20V). But as smartphones are space and height constrained (especially regarding the inductors used for switching converters) an increasing inductor current ripple (from higher input voltages) cannot be compensated with coils having higher inductance. As a result, either the DCR (DC resistance) of the coil may be increased or the switching frequency may be increased. Both measures lead to an increased dissipation power and may lead to hot spots at the housing of a battery powered application.